skill: add Android skills

.agents/skills/camerax/references/immutability.md

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+Many Android APIs are designed with immutability in mind to prevent race
+conditions in async environments. However, this often trips up developers used
+to mutable builder patterns.
+
+## Common immutable classes
+
+The following classes use fluent APIs that **return a new instance**. You must
+reassign the variable.
+
+| Class | Methods that return a new instance | Result if not reassigned |
+|---|---|---|
+| `PendingRecording` | `withAudioEnabled`, `asPersistentRecording` | Audio isn't recorded. |
+| `ImageCapture.Builder` | `setTargetRotation`, `setTargetResolution` | The output has the wrong orientation. |
+| `Recorder.Builder` | `setQualitySelector`, `setExecutor` | The recording uses the default quality. |
+| `Viewport.Builder` | `setScaleType`, `setLayoutDirection` | The viewfinder is stretched. |
+
+## Use standard patterns
+
+### CameraX video recording
+
+To set up video recording, use the following code:
+
+
+```kotlin
+// WRONG
+run {
+  val pending = recorder.prepareRecording(context, opts)
+  pending.withAudioEnabled() // This returns a new instance which is ignored
+  val active = pending.start(exec, listener)
+}
+
+// CORRECT
+run {
+  val pending = recorder.prepareRecording(context, opts)
+      .withAudioEnabled() // Chaining works
+  val active = pending.start(exec, listener)
+}
+
+// ALSO CORRECT
+run {
+  var pending = recorder.prepareRecording(context, opts)
+  pending = pending.withAudioEnabled() // Reassignment
+  val active = pending.start(exec, listener)
+}
+```
+
+<br />
+
+### Viewport construction
+
+To set up the viewport, use the following code:
+
+
+```kotlin
+val viewport = ViewPort.Builder(Rational(width, height), displayRotation)
+    .setScaleType(ViewPort.FILL_CENTER)
+    .build()
+```
+
+<br />